1. Field of the Invention
The invention relates generally to precision casting of molten material by means of methods and apparatus useful in the manufacture of artificial teeth, industrial mechanical parts and the like. The invention relates particularly to a method and apparatus for controlling current level in the arc discharge melting of material which is to be cast.
2. Description of the Prior Art
In the practice of precision casting, a material to be cast must be first melted, a common practice in the art being to melt such material by means of an arc discharge applied to the material. While arc discharge melting has a number of advantages, a problem typically encountered in this mode of melting is the stabilization of the arc discharge, it being difficult using prior art methods and arc discharge apparatus to stabilize the discharge due to the inability to stabilize the applied current. In conventional arc discharge melting devices which are presently available, arc discharge is at least partially stabilized by utilizing an arc power device which operates at a voltage which is approximately three times as great as the voltage necessary for arc discharge, the voltage then being reduced with a series of resistances or inductances. However, the use of substantially higher voltage levels than are necessary does not result in complete stabilization of the arc, it also not being possible with such prior art methods and apparatus to decrease electric input. Further, the high voltage levels required in such prior art methods necessitate the use of arc power apparatus of extremely high capacity, thereby increasing the expense of the total process. While an arc discharge can be more completely stabilized as arc power apparatus of increasingly higher voltages are utilized, the pursuit of such a solution to the problem is impractical due to the expense of the apparatus and the increasing size thereof.
The prior art also encompasses other methods and apparatus for stabilizing an arc, including the detection of arc current and subsequent adjustment of the alternating current of the applied input power by means of a thyristor. In this method and apparatus for arc current stabilization, the thyristor is "on" when an arc is started, thereby resulting at the instant of arc initiation in a flow of "over" arc current. The thyristor controls striking timing by detection of this over current. However, due to the transient over current, molten casting metal is caused to be vaporized and spattered with a strong potential for damage to the arc power circuit due to the shock thus resulting. As a consequence of this shock, the wave of the alternating current from the power input is disturbed and the resulting asymmetrical current flows in the primary coil of the power input transformer to cause a deviated magnetization such as would be caused by a direct current flowing in the primary coil. As a result, magnetizing control is broken at the peak voltage and a run away current is generated. In prior apparatus of this type, even the temporary generation of a run away current results in severe damage to the primary coil of the power input transformer and to the thyristor itself. Attempts to improve this thyristor apparatus have included the provision of a thyristor on a circuit more near the arc, the thyristor normally being on the dropping transformer. However, such a practice requires the use of large and extremely expensive thyristors due to the flow of large current levels in circuitry near the arc due to the large drops in voltage which are experienced. Further, thermal losses with such an arrangement are experienced and the use of a radiator is necessary for protection of the apparatus. As a further negative consequence, the total efficiency of the arc power device is lessened, thereby requiring an increase in the power necessary for striking an arc signal of the thyristor. An arc current control device utilized in such an environment must also be of large capacity.
The present invention intends solution to the problems encountered in the art as evidenced above through use of a method and apparatus capable of initiating arc current at a first lower level and rapidly and smoothly increasing the arc current level up to the steady higher current necessary for melting. The current level is increased immediately after starting of the lower current arc discharge to the steady higher current, this process preventing the occurrence of starting shock. The invention thus makes possible the generation of a stable arc discharge for melting casting materials smoothly through the use of a small and relatively compact direct current generating source.